Process for preparing purine nucleosides

ABSTRACT

The present invention for the stereoselective preparation of 2-deoxy-β-D-adenine nucleosides wherein a blocked 2-deoxy-α-D-arabinofuranosyl halide is coupled with the salt of an adenine derivative.

This application is a continuation-in-part of U.S. application Ser. No.10/209,808, filed Aug. 1, 2002 now U.S. Pat. No. 6,680,382, which claimspriority to U.S. provisional application Ser. No. 60/309,590, filed Aug.2, 2001, both of which are hereby incorporated by reference.

RESEARCH AGREEMENTS

The claimed inventions were made by, on behalf of, and/or in connectionwith: (i) a joint research agreement between Southern Research Instituteand Bioenvision, Inc., (f/k/a/Eurobiotech Group, Inc.), now part ofGenzyme Corporation, and (ii) a joint research agreement betweenBioenvision, Inc. and Ilex Oncology, Inc., now part of GenzymeCorporation. The agreements were in effect on or before the date theclaimed inventions were made, and the claimed inventions were made as aresult of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates generally to the chemical preparation ofpurine nucleosides. More specifically, the invention relates to thecoupling of an adenine derivative with a blocked arabinofuranosyl toform a β-D-adenine nucleoside derivative. Such nucleosides are valuablecompounds in the field of cancer therapy and as anti-viral agents.

BACKGROUND OF THE INVENTION

A number of β-D-purine nucleosides derived from adenine are useful asantitumor and antiviral agents. An important step in the synthesis ofsuch agents is the formation of the N-glycoside bond between the adeninenucleobase and an arabinofuranosyl derivative. The coupling reactionsused to form the N-glycoside bond of 2′-deoxynucleosides have typicallyresulted in the formation of a mixture of α and β-anomers.

Nucleosides have been synthesized by fusion glycosylation, wherein thereaction is carried out in the absence of solvent at a temperaturesufficient to convert the reactants to a molten phase. E.g.,2,6-dichloropurine has been coupled under fusion conditions with5-O-benzyl-2-deoxy-1,3-di-O-acetyl-2-fluroarabinose to form a2′-fluoroarabinonucleoside in 27% yield (Wright et al., J. Org. Chem.34:2632, 1969). Another synthetic method utilizes silylated nucleobasederivatives, e.g., a silylated nucleobase has been coupled with aperacetylated deoxy-sugar in the presence of a solvent and a FriedelCrafts catalyst (Vorbruggen et al., J. Org. Chem. 41: 2084, 1976). Thismethod has been modified by incorporating a sulfonate leaving group inthe deoxy-sugar in the synthesis of 2′-deoxy-2′-difluoronucleosides(U.S. Pat. Nos. 4,526,988; 4,965,374).

High yields of 2′-deoxy-2′-fluoro-pyrimidine nucleosides were obtainedfrom refluxing pyrimidines with2-deoxy-2-fluoro-3,5-di-O-benzoyl-α-O-arabinofuranosyl bromide. (Howellet al., J. Org. Chem. 53:85-88, 1988). It was found that use of solventswith lower dielectric constants produced have higher β:α anomer ratios.It was postulated that such solvents favored an S_(N)2 reaction, whereassolvents with higher dielectric constants favored production ofα-anomers via an ionic S_(N)1 pathway.

Anion glycosylation procedures have also been used to prepare2′-deoxy-2′-fluoropurine nucleosides. EP 428109 discloses the couplingof the sodium salt of 6-chloropurine, formed by sodium hydride, with3,5-dibenzyl-α-D-arabinofuranosyl bromide using conditions that favorS_(N) 2 displacement. Use of 1:1 acetonitrile/methylene chlorideresulted in a nucleoside product with a β:α anomer ratio 10:1, asopposed to a ratio of 3.4:1 observed when using a silylated purinereactant. In regard to the use of adenine salts, the amino substituentat the C-6 position was protected as a benzoyl derivative during thecoupling reaction. Protecting the exocyclic amino group precludes theformation of arabinofiiranosyl adducts which otherwise may be expectedto be produced (e.g., Ubukata et al., Tetrahedron Lett., 27:3907-3908,1986; Ubukata et al., Agric. Biol. Chem., 52: 1117-1 122, 1988; Searleet al., J. Org. Chem., 60:4296-4298, 1995; Baraldi et al., J. Med.Chem., 41:3174-3185, 1998). The preparation of α and β anomers of2′-deoxy-2′-fluoropurine and 2′-difluoropurine nucleosides by anionglycosylation are disclosed by U.S. Pat. Nos. 5,744,597 and 5,821,357with β-anomer enriched nucleosides prepared in a β:α anomer ratio ofgreater than 1:1 to about 10:1 and from greater that 1:1 to about 7:1respectively. In regard to purines substituted with exocyclic aminogroups, both patents again disclose protecting such groups duringcoupling to an appropriate sugar moiety. U.S. Pat. No. 5,821,357 alsodiscloses the effect of solvents on the β:α anomer ratio of 9-[1-(2′-deoxy-2′,2′-difluoro-3′,5′-di-O-benzoyl-D-ribofuranosyl)]-2,6-dipivalamidopurineprepared by coupling the potassium salt of 2,6-dipivalamidopurine withan α anomer enriched preparation of2-deoxy-2,2-difluoro-D-ribofuranosyl-3,5-dibenzoyl-1-trifluoromethanesulfonate. There was no correlation between thedielectric constant of the six solvents used and the β:α anomer ratio,e.g. ethyl acetate and acetonitrile both gave the same ratio of 1.6:1. t-Butyl alcohol gave the highest β:α anomer ratio of 3.5:1.

Despite the preparative methods for purine nucleosides known in the art,there is still a need for economically preferable, effective andefficient process for the preparation of these compounds. The object ofthe present invention is to provide such a process. Further objects areto minimize the number of process reaction steps and to provide aprocess that is readily scalable for the production of commercial-scalequantities. Other objects and advantages will become apparent to personsskilled in the art and familiar with the background references from acareful reading of this specification.

SUMMARY OF THE INVENTION

In its most general terms, one aspect of the present invention providesfor the preparation of β-adenine nucleosides by coupling an adeninederivative containing an unprotected exocyclic amino group at the C-6position, and a blocked arabinofuranosyl derivative. In preferredembodiments, this reaction can be depicted as:

R¹ is hydrogen, halogen or —OR⁶, wherein R⁶ is a hydroxy protectinggroup. In a preferred embodiment R¹ is fluoro. R² and R³ arehydroxy-protecting groups. In preferred embodiments R², R³ and R⁶ areindependently benzoyl or acetyl. R⁴ is a leaving group. Suitable leavinggroups include, halo, fluorosulfonyl, alkylsulfonyloxy,trifluoroalkylsulfonyloxy and arylsulfonyloxy. In a preferredembodiment, R⁴ is bromo. R⁵ is hydrogen, halogen or —NH₂. In preferredembodiments, R⁵ is chloro or fluoro.

Surprisingly, this reaction proceeds without substantial production ofadducts resulting from addition of the blocked arabinofuranosyl (1) withthe exocyclic amino group at the C-6 position of compound (2)(hereinafter termed “C-6 exocyclic amino group”), which remainsunprotected during the reaction, and/or the nitrogen at the N-7 positionof the adenine ring. An example of an undesired C-6 exocyclic aminogroup by-product adduct is represented by the following formula:

For the purposes of the present invention, and in light of the objectiveto provide an economically preferable, effective and efficient process,“substantial formation” means conversion of about 40% of the adeninederivative of formula (2) to a by-product adduct or adducts resultingfrom addition of the blocked arabinofuranosyl of formula (1) to theunprotected C-6 exocyclic amino group and/or N-7 position of compound(2). In embodiments wherein R⁵ is —NH₂ (hereinafter termed “R⁵ —NH₂group”), “substantial formation” means conversion of about 40% of theadenine derivative of formula (2) to by-product adduct(s) resulting fromaddition of the blocked arabinofuranosyl of formula (1) to theunprotected C-6 exocyclic amino group and/or N-7 position and/or the R⁵—NH₂ group of compound (2).

Even more surprising is that the reaction can proceed without even asignificant production of adducts resulting from addition of the blockedarabinofuranosyl (1) with the C-6 exocyclic amino group and/or N-7position of compound (2). For the purposes of the present invention,“significant production” means conversion of about 5% of the adeninederivative of formula (2) to a by-product adduct or adducts resultingfrom addition of the blocked arabinofuranosyl (1) to the unprotected C-6exocyclic amino group and/or N-7 position of compound (2). Inembodiments wherein R⁵ is —NH₂, “significant production” meansconversion of about 5% of the adenine derivative of formula (2) to aby-product adduct(s) resulting from addition of the blockedarabinofuranosyl of formula (1) to the unprotected C-6 exocyclic aminogroup and/or N-7 position and/or the R⁵ —NH₂ group of compound (2).

Useful bases are generally those with a pKa in water of 15 or greater.In preferred embodiments, the base is an alkali metal base with a pKa inwater of 17 or above, more preferred being a potassium base. Inpreferred embodiments, the base is a sterically hindered base, e.g.,potassium t-butoxide or potassium t-amylate. Suitable inert solventsinclude, but are not limited to, t-butyl alcohol, acetonitrile,dichloromethane, dichloroethane, t-amyl alcohol, tetrahydrofuran ormixtures thereof. In preferred embodiments, the solvent or solventmixture has a boiling point of about 80° C. or greater.

The process of the present invention also further comprisesde-protection of the blocked carbohydrate moiety to form a β-nucleosideof the formula:

wherein, R¹ and R⁵ are as defined above.

In some embodiments, the adenine derivative is 2-chloroadenine and theblocked arabinofuranosyl derivative is a2-deoxy-2-fluoro-arabinofuranosyl derivative, whereupon the resultingβ-nucleoside is a 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine derivative. The reaction can be depicted as:

wherein R², R³ and R⁴ are as defined above. The process also furthercomprises de-protecting the carbohydrate moiety to form2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine, also knownas clofarabine.

Another aspect of the invention is the discovery of the surprisingsteroselectivity that can be achieved in the production2′-deoxy-2′-halo-β-D-adenine nucleosides wherein such nucleosides arealso produced in high yield. This reaction can be depicted as:

R⁷ and R⁸ are independently hydrogen, C₁-C₄ alkyl, or a protectinggroup, R⁹ and R¹⁰ are independently halogen, M⁺ is potassium, R¹¹ ishalogen or —NR⁷R⁸, wherein R⁷ and R⁸ are as described above, and R² andR³ are as defined above. Halogen includes bromo, fluoro, chloro andiodo. In a preferred embodiment R¹⁰ is fluoro. In various embodiments R⁹is chloro or, preferably, bromo. In preferred embodiments, R⁷ and R⁸ areindependently hydrogen, acyl, such as acetyl or benzoyl, or silyl, suchas trimethylsilyl. In some embodiments, the process further comprisesthe addition of calcium hydride. Suitable substantially anhydrous inertsolvents include t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, tetrahydrofuran or mixtures thereof. Inpreferred embodiments, the substantially anhydrous solvent is a mixtureof t-butyl alcohol and acetonitrile, or a mixture of t-butyl alcohol anddichloroethane, or a mixture of dichloroethane and acetonitrile, or amixture of t-amyl alcohol and dichloroethane, or a mixture of t-amylalcohol and acetonitrile, or a mixture of t-amyl alcohol, acetonitrileand dichloromethane, or a mixture of t-amyl alcohol, acetonitrile anddichloroethane. In preferred embodiments, the substantially anhydroussolvent has a boiling point of about 80° C. or greater.

In some embodiments, the adenine derivative salt (10) is formed in situby the reaction of a potassium base with the corresponding adeninederivative. In some embodiments the potassium base has a pKa in water of15 or greater and in preferred embodiments the potassium base is ahindered base with a pKa in water of 17 or greater, such as potassiumt-butoxide or potassium t-amylate.

In preferred embodiments the reaction mixture is heterogenous in thateither: (1) the adenine salt, if added directly to the reaction mixture;or (2) the adenine base, if adenine salt is formed in situ in thereaction mixture, is not totally soluble in the substantially anhydroussolvent.

In various embodiments of the invention, the coupling reaction producesin the substantially anhydrous solvent reaction mixture, without furtherpurification or isolation, a preparation wherein the ratio of theβ-anomer of formula (11) to the α-anomer of formula (12) is at leastabout 10:1, or preferably is at least about 15:1, or more preferable isat least about 20:1. Thus, the anomer ratio may be 10:1 or greater, 15:1or greater or 20:1 or greater. In preferred embodiments the β-anomer offormula (11) is prepared in a yield of about 40% or greater. In morepreferred embodiments, the β-anomer of formula (11) is prepared inyields of about 50% or greater or about 80% or greater.

The process of the present invention may also further comprisesisolation of the β-anomer (11) by subjecting the mixture of β andα-anomers to recrystallization or by a re-slurry procedure. In apreferred embodiment, the further purification comprises reslurry frommethanol or crystallization from a mixture of butyl acetate and heptane.In various embodiments, the purified preparation comprises a mixture ofnucleosides wherein the ratio of the β-anomer of formula (11) to theα-anomer of formula (12) is at least about 20:1, or least about 40:1, orat least about 60:1.

The process also further comprises de-protection of the blockedcarbohydrate moiety of the protected β-anomer to form a β-nucleoside ofthe formula:

wherein, R⁵ and R¹⁰ are as defined above. When R⁵ is chloro and R⁸ isfluoro, the unblocked β-nucleoside of formula (13) is2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine. Thedeprotection process including the associated work-up of the product mayresult in a ratio of the β-anomer of formula (11) to the α-anomer offormula (12) that is at least about 99:1 or greater, or about 400:1 orgreater, or about 500:1 or greater, or about 1000:1 or greater.

Another aspect of the present invention is a multi-step process for thepreparation of a composition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine. Thiscomprises the integration of the other aspects of the present inventioninto an economically preferable, effective and efficient synthesis andisolation of 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine. This process minimizes the number of steps in part by notrequiring protection of the C-6 exocyclic amino group. In addition, thesurprising stereoselective preference for the β-anomer in part enablesthe preparation of a composition with an β:α anomer ratio of at least99:1 or in preferred embodiments is about 400:1 or greater, about 500:1or greater or about 1000:1 or greater, without utilizing a preparativechromatography step for the purification of the β-anomer. The absence ofa chromatographic step is a major advantage in regard to an economicallypreferable commercial-scale process.

The process comprises reacting3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide with a2-chloroadenine potassium salt of the formula:

in the presence of a substantially anhydrous solvent to form2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine. The C-6 exocyclic amino group of the 2-chloroadenine potassiumsalt is not protected during the process. The2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine is then de-protected to form2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine, which isthen isolated to provide a composition comprising2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine. In someembodiments, wherein the composition produced by the multi-step process,as described above, also comprises2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine, the2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine issubstantially pure. For the purposes of the present invention,substantially pure 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine means that the ratio of β-anomer to α-anomer as measured by highpressure liquid chromatography and spectrophotometric analysis, is atleast 99:1.

The process may further comprise isolating the2-chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine before the deprotection step. In some embodiments, thisisolation may comprise reslurry and/or recrystallization, which may beeffected by use of methanol or by use of a mixture of butyl acetate andheptane. In other embodiments, the isolation of2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine alsocomprises recrystallization. In some embodiments, the recrystallizationis from methanol.

In some embodiments, the 2-chloroadenine potassium salt is prepared insitu by the reaction of a potassium base with 2-chloroadenine in asuitable substantially anhydrous inert solvent. In preferredembodiments, the base is potassium t-butoxide or potassium t-amylate.Suitable inert solvents include t-butyl alcohol, acetonitrile,dichloromethane, dichloroethane, t-amyl alcohol, tetrahydrofuran ormixtures thereof. In preferred embodiments, the substantially anhydroussolvent is a mixture of t-butyl alcohol and acetonitrile, or a mixtureof t-butyl alcohol and dichloroethane, or a mixture of dichloroethaneand acetonitrile, or a mixture of t-amyl alcohol and dichloroethane, ora mixture of t-amyl alcohol and acetonitrile, or a mixture of t-amylalcohol, acetonitrile and dichloromethane, or a mixture of t-amylalcohol, acetonitrile and dichloroethane.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1—Schematic representing potential rationale for the effect ofpotassium in the stereoselective production of2′-deoxy-2′-fluoro-β-D-adenine nucleosides. R², R³ and R⁵ are as definedin the Description.

FIG. 2—Schematic of expected conformations of the relevant protons andfluorine atoms for 2-chloro-9-(2′-deoxy-2′-halo-β-D-arabinofuranosyl)adenine (clofarabine) (21) and2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine(epi-clofarabine) (22).

FIG. 3—Partial 1H NMR for2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine(clofarabine) (21).

FIG. 4—Partial 1H NMR for2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine(epi-clofarabine) (22).

DETAILED DESCRIPTION OF THE INVENTION

1. Coupling Reactions Utilizing Purine Bases with Unprotected ExocyclicAmino Groups

One aspect of the present invention provides for the preparationβ-adenine nucleosides by coupling an adenine derivative with anunprotected C-6 exocyclic amino group and a blocked arabinofuranosylderivative, in the presence of a base and solvent. The blockedarabinofuranosyl derivative may be depicted by the structure:

R¹ is hydrogen, halogen or —OR⁶, wherein R⁶ is a hydroxy protectinggroup. Halogens include bromo, chloro, fluoro and iodo. R² and R³ arehydroxy protecting groups. Hydroxy protecting groups are known in theart as chemical functional groups that can be selectively appended toand removed from a hydroxy functionality present in a chemical compoundto render such functionality inert to chemical reaction conditions towhich the compound is exposed. Hydroxy protecting groups are describedin Greene and Wuts, Protective Groups in Organic Synthesis, 2d edition,John Wiley & Sons, New York, 1991, and include formyl, acetyl,propionyl, arylacyl (e.g., benzoyl or substituted benzoyl), trityl ormonomethoxytrityl, benzyl or substituted benzyl, carbonate derivatives(e.g., phenoxycarbonyl, ethoxycarbonyl and t-butoxycarbonyl), andtrisubstituted silyl, including trialkylsilyl (e.g.dimethyl-t-butylsilyl) or diphenylmethylsilyl. In preferred embodiments,the protecting groups are independently benzoyl or acetyl.

R⁴ is a leaving group, suitable examples of which include halogen,alkylsulfonyloxy, and arylsulfonyloxy. Halogens include chloro, fluoro,iodo and, in a preferred embodiment, bromo. Blocked α-arabinofuranosylhalides can be prepared by various methods known in the art employingstandard procedures commonly used by one of skill in the art, e.g.,3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide(exemplified in Example 1; Tann et al., J. Org. Chem., 50:3644, 1985,herein incorporated by reference);3-O-acetyl-5-O-benzyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (Foxet al., Carbohydrate Res., 42:233, 1975, herein incorporated byreference); 2,3,5-O-tribenzyl-α-D-arabinofuranosyl chloride (U.S. Pat.No. 5,110,919, herein incorporated by reference); and3,5-O-di-p-toluoyl-2-deoxy-α-arabinofuranosyl chloride (Bhattacharya etal., J. Org. Chem., 28:428 1963; Nuhn et al., Pharmazie, 24:237, 1969,both herein incorporated by reference). Preparation of blockedα-arabinofuranosyl derivatives substituted at the C-1 position withalkylsulfonates and arylsulfonates are disclosed in U.S. Pat. Nos.5,401,861 and 5,744,579, both herein incorporated by reference. Alkylsulfonates include methanesulfonate, ethylsulfonate and butylsulfonateand substituted alkyl sulfonates include compounds such astrifluoromethane sulfonate and 1,1,1-trifluoromethanesulfonate.Arylsulfonates includes substituted arylsulfonates such asp-nitrobenzenesulfonate, p-bromobenzenesulfonate,p-methylbenzesulfonate, and the like.

Useful bases generally have a pKa in water of 15 or greater and aresuitable for the formation of a salt of the adenine derivative (2), asdepicted by the formula:

R⁵ is as defined previously and R⁺ is a monovalent cation. The base maybe an alkali metal base, and in preferred embodiments the alkali metalbase is a potassium base. In preferred embodiments, the base is asterically hindered base, e.g., potassium t-butoxide or potassiumt-amylate.

Solvents useful in the present invention are those that are inert inrespect to the reaction. Suitable inert solvent include, but are notlimited to, t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, tetrahydrofuran or mixtures thereof.

In a preferred embodiment, the reaction is carried out at roomtemperature. However, in other embodiments the reaction is carried outat elevated or lower temperatures. E.g., the reaction can be carried outat about 40° C., or about 50° C., or about 60° C., or under refluxconditions. Alternatively the reaction can be carried out from about−25° C. to about 25° C., e.g., at about −20° C. or at about −10° C., orat about 0° C., or at about 10° C.

Wherein an amino group is described as “unprotected,” this means thatthe amino group has not been blocked by an amino protecting group. Theuse and types of amino protecting functionalities are well known in theart. Examples are described in Greene and Wuts, Protective Groups inOrganic Synthesis, 2d edition, John Wiley & Sons, New York, 1991.

The molar ratio of reactants is not considered to be critical and inpreferred embodiments approximately equal molar equivalents of blockedarabinofuranosyl derivative (1), adenine derivative (2) and base areused. In some embodiments, a slight molar excess (e.g., 1.05 to 1.15equivalents) of adenine derivative (2) and/or base are used. Thepreferred order and manner of addition for any specific embodiment canbe determined by routine experimentation with a view towards bothreaction performance and chemical engineering and productionsconsiderations.

2. Stereoselective Preparation of 2-Deoxy-Purine Nucleosides

Another aspect of the invention is the stereoselective preparation of2-deoxy-β-D-adenine nucleosides. In this process, a blocked2-deoxy-α-D-arabinofuranosyl halide is coupled with the salt of anadenine derivative depicted by the formula:

wherein R⁷, R⁸, R¹¹ and M⁺ are as previously described. Surprisingly,the identity of the cation has a profound effect on thestereoselectivity of the coupling reaction. Potassium salts producedlarger β:α anomer ratios than lithium or sodium salts. The salt depictedby formula (10) can be produced in situ by use of potassium bases andthe corresponding adenine derivatives. Suitable bases generally have apKa in water of 15 or greater and include potassium t-alkoxide bases,potassium hydroxide and hindered bases include potassiumdiisopropylamide, potassium bis(trimethylsilyl)amide, potassiumhexamethyldisilazide, potassium hydride and the like. In preferredembodiments, the base is a sterically hindered base with a pKa in waterof 17 or above, e.g., potassium t-butoxide or potassium t-amylate. Thestronger bases are preferred in that they enhance the production of thepotassium salt of the purine and also because there is in general areciprocal relationship between the strength of a base andnucelophilicty, and in that regard stronger bases have less of apropensity to displace the C1 halide of the sugar reactant. Hinder basesare preferred, at least in part, for a similar rationale, in thathindered bases have less of a propensity than non-hindered bases to actas a nucleophile. In addition, bases such as potassium t-butoxide andpotassium t-amylate, produce the corresponding alcohols during thereaction, solvents that do not have detrimental effect on the reactionand may have a beneficial effect on the reaction. In contrast, basessuch as potassium hydroxide produce water during the reaction, which asstated below, has a potentially detrimental effect on the reaction.

While not being bound by any theory, the preferential stereoselectivityobserved with potassium may be due, e.g., when R¹⁰ is fluoro and R⁹ isbromo, to an electrostatic attraction between the electronegativefluorine atom and the hard potassium cation, leading to a preferentialβ-face attack, as depicted in FIG. 1. The lack of selectivity of lithiumand sodium may be due to a more covalent association of the cation withthe purine base. The present invention also encompasses other cations,such as cesium, that can replace potassium as a hard cation.

The solvent employed also has a marked effect on the β:α anomer ratio.Generally solvents with a lower dielectric constant favor production ofthe β anomer. But solvent choice is not dictated simply by dielectricconstant, in that there is a tendency for an inverse relationshipbetween increasing the β:α anomer ratio and the yield of the β and αanomers. This effect presumably relates to the solubility of reactantsand/or intermediates. In this regard, in preferred embodiments, thereaction is heterogenous, meaning in the context either: (1) the adeninesalt, if added directly to the reaction mixture; or (2) the adeninebase, if adenine salt is formed in situ in the reaction mixture, is nottotally soluble in the substantially anhydrous solvent. These conditionsfavor combinations of desirable β:α anomer ratios and yields. Suitablesolvents include t-butyl alcohol, acetonitrile, dichloromethane,dichloroethane, t-amyl alcohol, isoamyl alcohol, tetrahydrofuran ormixtures thereof. In preferred embodiments, the solvent is a mixture oft-butyl alcohol and acetonitrile, or a mixture of t-butyl alcohol anddichloroethane, or a mixture of dichloroethane and acetonitrile, or amixture of t-amyl alcohol and dichloroethane, or a mixture of t-amylalcohol and acetonitrile, or a mixture of t-amyl alcohol, acetonitrileand dichloromethane, or a mixture of t-amyl alcohol, acetonitrile anddichloroethane. Two component mixtures the two solvents may be combinedin the range of about 1:4 to about 1:1 v/v. In three component mixtures,the three solvents may be combined in ratios of about 2:2:1, or about2:1:1, or about 1:1:1. In preferred embodiments the solvents aresubstantially anhydrous as water has been found to be detrimental to thedesired high β:α anomer ratio. Without being bound to any particulartheory, it has believed that the water coordinates to the potassiumcation thereby interfere with the proposed electrostatic interactionbetween the hard C-2 fluorine atom of the sugar and the hard potassiumcation. Substantially anhydrous solvents may be prepared by predryingsolvents, e.g., by use of calcium hydride, for use with strong hinderedbases such as potassium t-butoxide and potassium t-amylate. For use ofbases, such as potassium hydroxide, that produce water, a drying agent,such as calcium hydride, is preferentially present during the reactionto maintain the substantially anhydrous nature of the solvent. Thus invarious embodiments, drying agents such as calcium hydride may be usedto predry solvents and/or may be present in the reaction.

In a preferred embodiment, the reaction is carried out at roomtemperature. In other embodiments, elevated or lower temperatures areused. Lowering the temperature of the reaction, such as in the range offrom room temperature to about −25° C., may lead to an increase the β:αanomer ratio. Elevated temperatures can be used in the range from roomtemperature to reflux conditions.

The molar ratio of reactants is not considered to be critical and inpreferred embodiments when the adenine derivative salt (10) is producedin situ, approximately equal molar equivalents of blockedarabinofuranosyl derivative (9), the corresponding adenine derivative,base and, when added, calcium hydride are used. In some embodiments, aslight molar excess (e.g., 1.05 to 1.15 equivalents) of thecorresponding adenine derivative and/or base are used. The preferredorder and manner of addition for any specific embodiment can bedetermined by routine experimentation with a view towards both reactionperformance and chemical engineering and productions considerations.

EXAMPLES OF THE INVENTION

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following specific examples are intended merelyto illustrate the invention and not to limit the scope of the disclosureor the scope of the claims in any way whatsoever.

Example 1 Preparation of3,5-O-Dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl Bromide

A 1-neck roundbottom flask (100 mL) was equipped with a stir bar andnitrogen inlet adapter. The flask was charged with dichloromethane (10.4mL) and 1,3,5-O-tribenzoyl 2-deoxy-2-fluoro-β-D-arabinofuranosyl (16)(2.6 gm, Sigma, St. Louis, Mo.) at room temperature. The solution wasplaced under nitrogen. A 33% solution of hydrogen bromide in acetic acid(0.96 gm) was charged and the resultant mixture stirred for 18 hr. Thesolvent was removed by rotary evaporation to give an orange residue.This was dissolved in dichloromethane (30 mL) and quenched with sodiumbicarbonate brine (30 mL), whereupon the pH was 7-8. The organic phasewas partitioned and washed with sodium chloride brine (30 mL). Theorganic phase was dried over MgSO₄ and filtered. Solvent removal byrotary evaporation and high vacuum afforded3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) as aviscous yellow gum.

Example 2 Preparation of2-Chloro-9-(3,5-O-dibenzoyl-2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenine

2-Chloro-9-(3,5-O-dibenzoyl-2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenine (19) (Borregaard) was prepared utilizing different bases andnumerous solvent systems and the optional addition of calcium hydride.In the following exemplifications, three preparations are described indetail and other preparations are summarized in Table 1.

A. Preparation I

A three neck roundbottom flask was equipped with a temperaturecontroller, nitrogen inlet and outlet tubes, septa and a magnetic stirbar. Chloroadenine (18) (0.45 g) was charged as a solid under nitrogen,followed by potassium t-butoxide (0.34 g), acetonitrile (2.3 mL) andt-butyl alcohol (6.9 mL). After stirring for 1 hour at 24° C.-26° C.,3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) (1.21gm) was added. The resulting orange suspension was stirred at 24° C.-26°C. for 16 hours. HPLC analysis of an in-process control sample showed a96.6% conversion and a 10.7:1 ratio of β-anomers (19) to α-anomer (20).HPLC analysis utilized a reverse phase system with a Zorbax-SB-C18column and a mobile phase of 80:20 acetonitrile/water with 15% v/vtrifluoroacetic acid at a flow rate of 1 mL/min. at 30° C. Detection wasby spectrophotometric analysis at 263 nm. Conversion is expressed asarea under the curve (a.u.c.) values of (19)+(20)/(18)+(19)+(20)×100.The solvent was evaporated to afford 1.79 g of an orange residue. Tothis was added ethyl acetate (34 mL) and the mixture stirred at ambienttemperature for 1.25 hr and then filtered through filter paper and thepaper rinsed twice with 5 mL of ethyl acetate. Evaporation of thefiltrate solution afforded 1.28 g of light orange crystals (86.8% byHPLC area of the combined anomers). This material still contained asmall amount of 2-chloroadenine (13) by HPLC. The anomeric ratio was11.8:1. The crystals were dissolved with 33 mL ethyl acetate at ambienttemperature to afford a slightly opaque solution. This was filteredthrough filtered through a Celite pad and the filtrate evaporated toafford 1.16 g of crystals. This material still contained a small amountof (13). The problem was remedied by more efficient filtration. Thecrystals were dissolved in 25 mL ethyl acetate overnight at ambienttemperature to give a slightly cloudy solution. This was filteredthrough a Whatman 0.45 mM nylon syringe filter and evaporated to afford1.13 g. This material contained no (18) by HPLC analysis and had ananomeric ratio of 11.9:1 and a yield of 83% with a purity of 98.1%(a.u.c.). Considering the production of anomers (19) and (20), there wasno substantial formation of a by-product adduct formed by reaction of3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) withthe unprotected exocyclic amino group of 2-chloroadenine (18). Inaddition, HPLC analysis revealed no substantial formation ofby-products.

B. Preparation II

A 3-neck roundbottom flask was equipped with a magnetic stir bar,temperature controller, and nitrogen inlet line and charged with2-chloradenine (18) (0.29 g), followed by acetonitrile (1.6 mL), t-amylalcohol (3.3 mL), potassium tert-butoxide (0.2 g) and calcium hydride(0.069 g). This mixture was stirred at 25° C. for 30 minutes before3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) (0.68g gm) dissolved in dichloromethane (3.25 mL) was charged. The orangesolution was stirred for two days whereupon HPLC analysis showed a β:αanomeric ratio of 18.8:1 and a conversion of approximately 67%. Heatingat 40° C. for approximately 4.5 hr resulted in a β:α anomer ratio of18.7:1 and a decrease in the apparent conversion to 63%. The reactionmixture was vacuum filtered and the filter cake washed withdichloromethane (2×12 mL). The filtrate was passed through a nylonsyringe filter and then concentrated by rotary evaporation and highvacuum pumping to afford 0.72 g of material with a β:α anomeric ratio of19:1 and was 88% pure by HPLC (a.u.c.), giving a yield of the anomers(19) and (20) of 77%. In that there was an approximately 77% conversionof the chloroadenine, there was neither substantial nor significantformation of a by-product adduct formed by reaction of3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosyl bromide (17) withthe unprotected exocyclic amino group of 2-chloroadenine (18). Inaddition, HPLC analysis revealed no substantial nor significantformation of by-products.

C. Preparation III

A 3-neck 100 ml round-bottomed flask equipped with magnetic stir bar,temperature controller, and nitrogen inlet line and charged with 2:1t-amyl alcohol:acetonitrile (9 mL) followed by 2-chloradenine (18) (0.63g), potassium t-amylate (0.47 g) and calcium hydride (0.15 g). Thismixture was stirred at room temperature for 30 minutes before theaddition of 3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosylbromide (17) (1.5 gm) dissolved in 2:1 t-amyl alcohol:acetonitrile (7mL). The solution was stirred for 17 hr. whereupon analysis by HPLCshowed the conversion to be approximately 79% and a β:α anomer ratio of14.5:1. The reaction mixture was vacuum filtered and the residue washedwith 2×5 mL acetonitrile. The filtrate was re-filtered through a 0.45 μnylon filter and then concentrated. The concentrate residue wasdissolved in butyl acetate (5 mL). Heptane (35 mL) was added and theresulting crystals were collected by vacuum filtration and subjected toa high vacuum. HPLC analysis of the crystals indicated a β:α anomerratio of 19.4:1 and a 63% yield of material with a 90% purity (a.u.c.).In that there was an approximately 79% conversion of the chloroadenine,there was no substantial formation of a by-product adduct formed byreaction of 3,5-O-dibenzoyl-2-deoxy-2-fluoro-α-D-arabinofuranosylbromide (17) with the unprotected exocyclic amino group of2-chloroadenine (18). In addition, HPLC analysis revealed no substantialformation of by-products.

D. Summary of Preparative Methods

Results of preparative examples in addition to those exemplified abovein Preparations I, II and III, are summarized in Table 1. Preparativemethods typically used approximately molar equivalents of (17) and (18)and calcium hydride and a slight molar excess of base.

TABLE 1 Time β:α Ratio Conversion Isolated Solvent^(‡) Base* CaH₂ (hrs)(19)/(20) %^(†) Yield (%) 2:1 KOtBu + 14 17 54 ND^(††) tBuOH/DCE 1:2KOtBu + 14 20.1 60 ND DCE/tAmOH 1:4 KOtBu + 14 20.5 58 ND DCE/tAmOH1:2:2 KOtBu + 14 22.1 74 ND MeCN/DCE/ tAmOH 52% tBuOH KOtBu + 26 10.7 9042 48% MeCN 52% tBuOH KOtBu − 22 10.7 84 77 48% MeCN 52% tBuOH KOtBu −21 11 86 79 48% MeCN 51% amyl KOtBu − 17 13.1 80 83 alcohol 49% MeCN2:2:1 CH₂Cl₂:tAmOH:MeCN KOtBu + 85 18.7 71 80 2:1 KOtBu + 85 12.7 70 84tAmOH:MeCN 2:1 KOtBu + 85 13.1 77 89 tAmOH:MeCN 2:2:1 CH₂Cl₂:tAmOH:MeCNKOtBu + 69 13.9 73 41 2:1 tAmOH:MeCN K − 18 19.6 76 80 t-amylate 1:1t-AmOH:MeCN K − 18 13.3 79 84 t-amylate 1:2 tAmOH:MeCN K − 18 6.73 92 NDt-amylate 2:1:1 tAmOH:MeCN:CH₂Cl₂ KOtBu + 16 20.3 79 48 ^(‡)tBuOH =t-butyl alcohol; DCE = dichloroethane; tAmOH = t-amyl alcohol; MeCN =acetonitrile. *KOtBu = potassium t-butoxide. ^(†)Conversion % = a.u.c.of (19) + (20)/(18) + (19) + (20) × 100. ^(††)ND = not determined.

Example 3 Purification of2-Chloro-9-(3,5-O-dibenzoyl-2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenineby Re-slurry

A re-slurry step utilizing methanol reflux was used to purify compound(19). I necessary, the pH should be adjusted to 6.0 prior to this stepto prevent deprotection during the re-slurry step. Given that there-slurry must involve an equilibrium between the solid and solutionphases, a period of time is required for this equilibrium to becomeestablished under a given set of experimental conditions. Thus, thetimes required for equilibration by monitoring the anomeric compositionof slurries at different solvent ratios and temperatures were examined.Three salient features became apparent: (1) a hot re-slurry resulted ingreater amounts of (19) in the solution at equilibrium; (2) the amountof (19) in solution phase increases over time as equilibrium isapproached for the hot re-slurry and decreases over time for a roomtemperature re-slurry; and (3), equilibrium is essentially achieved at 5hours under hot or room-temperature re-slurry conditions, although aslight change is observed under room temperature conditions overovernight stirring. The room temperature re-slurry produced a greateranomeric increase. It was concluded that a re-slurry at roomtemperature, for at least 5 hours, followed by a 1 hour cooling andfiltration results in the best recovery and anomeric ration. Results ofthis method are shown in Table 2 for 20 gm runs undertaken in a 1 Lreactor.

TABLE 2 Initial ratio Final ratio Conditions^(†) (19)/(20)^(‡) (19)/(20)Mass recovery* A 19 79 62 B 20 39 69 B 24 66 74 ^(†)Conditions: A: 10 mlMeOH per gram of crude (19), reflux 0.5 hour then room temperature for19 hours, B: room temperature for 5 days. ^(‡)refers to the anomericratio going into methanol re-slurry step. *refers to the mass recoveryin the methanol re-slurry step only.

Example 4 Hydrolysis of Condensation Product to Afford2-Chloro-9-(3′,5′-O-dibenzoyl-2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine

Because methyl benzoate is a liquid and is readily soluble in manyorganic solvents, cleavage of benzyl groups with sodium methoxide waspreferred. A 250 ml, multi-neck flask, equipped with a thermocouple,magnetic stirrer, nitrogen purge and reflux condenser, was charged with(19) (8.42 gm, 16.45 mmol) and 15 ml methanol at ambient temperature.Stirring was started ands the mixture heated to 38° C. The reaction wascharged with sodium methoxide (62 μl, 0.329 mmol). The reaction mixturewas stirred at 38° C. for 7 hours, heating was them shut off and themixture cooled to ambient temperature and stirred overnight. The pH wasadjusted to 5.0 with acetic acid. The reaction flask was cooled in anice bath 2 hours and the reaction mixture was filtered and the flask andfiltercake were washed with 9.5 ml methanol. The wet solid and 105 mlmethanol were charged to a 250 ml, multi-neck flask, equipped with athermocouple, magnetic stirrer, nitrogen purge and reflux condenser,stirred and heated to reflux. The hot solution was filtered and filtratetransferred to the original reaction flask, wherein the mixture wascooled to ambient temperature. The mixture was cooled in and ice/waterbath for 0.5 hour and the mixture filtered and flask and filtercakerinsed with 9.8 ml 10 methanol. The wet solid was dried in a vacuum ovento produced (21) at a yield of 69.4% with a purity of 99.14 (a.u.c.). Noα-amoner was detectable by HPLC

Further examples of the deprotection method with varying conditions areshown in Table 3.

TABLE 3 MeOH % Mass Anomeric HPLC mmol (19) NaOMe eq. mL/g Temp. ° C.Recovery Ratio Area (%) 1.68 0.015 4 25 68.8 338/1 98.0 2.05 0.100 20 2558.9 415/1 99.6 2.11 0.010 20 25 58.4 469/1 93.7 2.09 0.010 4 25 64.2248/1 99.0 2.03 0.100 4 25 68.2 126/1 98.6 2.82 0.055 12 38 60.2 330/199.1 3.07 0.055 12 38 66.9 521/1 99.0 2.94 0.055 12 38 64.9 ∞/1 99.62.01 0.100 4 50 62.6 1657/1 99.4 2.07 0.010 4 50 64.3 521/1 98.9 1.970.010 20 50 59.3 432/1 99.4 1.99 0.100 20 50 61.0 397/1 61.0 17.05 0.0208 25 53.5 988/1 98.8

Example 5 NMR Designations for Clofarabine and Epi-Clofarabine

Pooled preparations of anomeric mixtures of (19) and (20) were pooledand de-protected by removal of the benzoyl groups by treatment withsodium methoxide and methanol. The resulting clofarabine andepi-clofarabine were isolated by preparative HPLC. In a typical run, 60mg of crude sample was dissolved in 1.4 mL of the mobile phase, i.e. 1:9(v/v) acetonitrile/water, for injection onto a Phenomenex Progidy C18,10 μ ODS, 250×21.2 mm column and a flow rate of 12 mL/min. Pooledfractions were rotary evaporated to remove acetonitrile and lyophilized.Purified samples were subjected to NMR analysis.

FIG. 2 shows the expected conformations of the relevant protons andfluorine atoms for 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl)adenine (clofarabine) (21) and2-chloro-9-(2′-deoxy-2′-fluoro-α-D-arabinofuranosyl) adenine(epi-clofarabine):

Based on these conformational assumptions and the Karpus relationship,the predicted coupling constants of the β-anomer (21) and the α-anomer(22) should conform to the following relationship:

a) JH₂F will be large for both the β or α anomers

b) (JH₁F)_(β)<(JH₁F)_(α)

c) (JH₁H₂)_(β)>(JH₁H₂)_(α)

d) JH₂H₃ will be small for both the β or α anomers

These predictions are borne out by the NMR analysis of the purifiedanomers as shown in Table 2, FIG. 3 and FIG. 4. Notably, the exocyclicN₆ protons occur at a predictable chemical shift (7.8-8.0 ppm) forclofarabine (21) and epi-clofarabine (22). Similar N₆ chemical shiftswere reported for other adenine derivatives (Reid et al., Helv. Chim.Acta, 72:1597-1606, 1989).

TABLE 2 Relevant Chemical Shifts and Coupling Constants for Anomers^(†)Compound H₂ δ (ppm) H₁ δ (ppm) Clofarabine 5.76 (dt, 1H, J = 63 6.31(dd, J = 15 Hz, J = 5 Hz) Hz, J = 5 Hz) Epi-Clofarabine 5.61 (dt, J = 57Hz, 6.19 (dd, J = 19.5 Hz, J = 4 Hz) J = 4 Hz) ^(†)HNMR data collectedat 250 MHz in DMSO-d6

The present invention has been shown by both description and examples.The Examples are only examples and cannot be construed to limit thescope of the invention. One of ordinary skill in the art will envisionequivalents to the inventive process described by the following claimsthat are within the scope and spirit of the claimed invention.

1. A process for the stereoselective preparation of a2′-deoxy-β-nucleoside of the formula:

wherein: R² and R³ are independently hydroxy protecting groups, R⁷ andR⁸ are independently hydrogen, C₁-C₄ alkyl, or amino protecting groups,and R¹¹ is a halogen or —NR⁷R⁸, wherein R⁷ and R⁸ are as describedabove; comprising reacting a 2-deoxy-α-arabinofuranosyl derivative ofthe formula:

wherein R⁹ is a halogen and R² and R³ are as defined above, with anadenine derivative potassium salt of the formula:

wherein: R⁷, R⁸ and R¹¹ are as defined above, in the presence of asubstantially anhydrous solvent, wherein the substantially anhydroussolvent is a mixture of t-amyl alcohol, acetonitrile and dichloroethaneor a mixture of t-amyl alcohol, acetonitrile and dichloromethane,wherein the ratio of solvents in said solvent mixture of t-amyl alcohol,acetonitrile and dichloroethane is about 2:1:1 by volume, respectively,or the ratio of solvents in said solvent mixture of t-amyl alcohol,acetonitrile and dichloromethane is about 2:1:1 by volume, respectively,and wherein said 2′-deoxy-β-nucleoside is produced in said substantiallyanhydrous solvent in a molar ratio of greater than 10:1 relative to the2′-deoxy-α-nucleoside anomer represented by the formula:


2. The process of claim 1, wherein said molar ratio of said2′-deoxy-β-nucleoside to said 2′-deoxy-α-nucleoside is at least 15:1. 3.The process of claim 1, wherein said molar ratio of said2′-deoxy-β-nucleoside to said 2′-deoxy-α-nucleoside is at least 20:1. 4.The process of claim 1, wherein R⁹ is bromo or chloro.
 5. The process ofclaim 1, wherein R¹¹ is chloro.
 6. The process of claim 1, wherein R²and R³ are independently benzyl or acetyl.
 7. The process of claim 1,wherein R⁷ and R⁸ are independently hydrogen, acetyl, benzoyl, ortrimethylsilyl.
 8. The process of claim 1, wherein said adeninederivative salt is formed in situ in said solvent by the reaction of apotassium base of a pKa in water of 15 or above, with an adeninederivative of the formula:

wherein the reaction mixture is heterogeneous in that said adenine baseis not totally soluble in said solvent.
 9. The process of claim 8,wherein said potassium base is a hindered potassium base with a pKa inwater of 17 or above.
 10. The process of claim 9, wherein said hinderedpotassium base is potassium t-butoxide or potassium t-amylate.
 11. Theprocess of claim 1, further comprising calcium hydride.
 12. The processof claim 1, further comprising purification of said β-nucleoside byrecrystallization or preparation of a slurry in an inert solvent. 13.The process of claim 1, wherein said purification of said β-nucleosidecomprises reslurry from methanol.
 14. The process of claim 1, furthercomprising de-protecting said 2′-deoxy-β-nucleoside of the formula:

to form a 2′-deoxy-β-nucleoside of the formula:

wherein R⁵ is a halogen or —NH₂.
 15. A process for the stereoselectivepreparation of a 2′-deoxy-β-nucleoside of the formula:

wherein R² and R³ are independently hydroxy protecting groups,comprising reacting a 2′-deoxy-α-arabinofuranosyl derivative of theformula:

wherein R² and R³ are independently hydroxy protecting groups, with2-chloroadenine, in the presence of a substantially anhydrous solvent,and a hindered potassium base with a pKa in water of 15 or above,wherein said substantially anhydrous solvent is a mixture of t-amylalcohol, acetonitrile and dichloroethane or a mixture of t-amyl alcohol,acetonitrile and dichloromethane, and wherein the ratio of solvents insaid solvent mixture of t-amyl alcohol, acetonitrile and dichloroethaneis about 2:1:1 by volume, respectively, or the ratio of solvents in saidsolvent mixture of t-amyl alcohol, acetonitrile and dichloromethane isabout 2:1:1 by volume, respectively, and wherein said2′-deoxy-β-nucleoside is produced in a molar ratio of greater then 10:1relative to the 2′-deoxy-α-nucleoside anomer represented by the formula:


16. The process of claim 15, wherein said molar ratio of said2′-deoxy-β-nuclcoside to said 2′-deoxy-α-nucleoside is at least 15:1.17. The process of claim 15, wherein said molar ratio of said2′-deoxy-β-nucleoside to said 2′-deoxy-α-nucleoside is at least 20: 1.18. The process of claim 15, wherein said 2′-deoxy-α-arabinofuranosylderivative and said 2-chloroadenine are reacted in the presence ofcalcium hydride, the substantially anhydrous solvent, and the hinderedpotassium base.
 19. The process of claim 15, wherein R² and R³ areindependently benzyl or acetyl.
 20. The process of claim 15, whereinsaid hindered potassium base has a pKa in water of 17 or above.
 21. Theprocess of claim 20, wherein said potassium base is potassium t-butoxideor potassium t-amylate.
 22. The process of claim 15, further comprisingpurification of said β-nucleoside by recrystallization or preparation ofa slurry in an inert solvent.
 23. The process of claim 22, wherein saidpurification of said β-nucleoside comprises reslurry from methanol. 24.The process of claim 22, further comprising de-protecting said2′-deoxy-β-nucleoside of the formula:

to form a 2′-deoxy-α-nucleoside of the formula: